This project, linked to another R01 entitled "Small Town IDU networks and HIV: transition dynamics" through the IRPG mechanism, has the primary aim of elucidating the dynamic structure of social networks of persons who use drugs and engage in unsafe sexual practices in a disadvantaged community in Atlanta, GA. Specifically, the project will investigate the change in social network structure over time, the influence of changing structure on HIV transmission, and explore the potential for network-based intervention strategies. We will recruit 60 index cases (half using a random walk technique and half using a modified chain-linking procedure). From each group of 30, we will use systematic sampling (every third respondent) to select 10 for full network ascertainment, for a total of 260 respondents (range, 220-300) and identification of 2660 contacts (range 1820-3660). Bach respondent will be interviewed and assessed serologically four times at 6-month intervals. The formal network ascertainment will be accompanied by ongoing field ethnography. We will use ethnographic analysis, and combined network and epidemiologic analysis to investigate a number of hypothesis that focus on the role of network structure as an obstacle or facilitator to HIV transmission; on the assessment of the relative position of the community in the spectrum of HIV transmission based on clinical parameters; on rapid methods for network ascertainment; and on the influence of cultural components on potential transmission. The results of this project, whose design is identical to that of the companion R01 in this IRPG, will be amplified by co-analysis with another community' that differs in some of its demographic characteristics and in its place along the spectrum of HIV prevalence. The cross-community comparison will provide insight into the relationship of networks and HIV prevalence. Repeated interviews over a 24 month period will provide insight into the dynamics of network transition and its possible influence on HIV transmission.